Author Topic: Simplification of Bariums SnCl2 ketone procedure  (Read 8972 times)

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armageddon

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partial vapor pressure
« Reply #20 on: May 30, 2004, 10:21:00 PM »
Starlight: Isn't the vapor pressure the sum of the partial vapor pressures of all boiling liquids involved (and total vapor pressure = atmosphere pressure)? And is the vapor pressure of water not lowered much less than the vapor pressure of the higher boiling liquid if atmosphere pressure is reduced? (my nomograph wants to tell me same vac causes different bp lowering, depending on substance/original bp - and bp depends on vapor pressure I think. So same vac will cause different changes of the partial vapor pressures, but maybe proportions of the partial pressures stay the same?)

I begin to see why someone invented the "evap enthalpy" thingy...

Hm. (maybe I shoud've opened a new thread, too late now)

Greetz A


Nicodem

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Vacuum steam distillation of P2P
« Reply #21 on: May 31, 2004, 12:19:00 PM »
Vacuum steam distillation of P2P - A theoretical enquiry

In order to fully understand the influence of pressure on the steam distillation phenomena I decided to play a little with the Claussius-Clapeyron's equation:

ln(P2/P1)=(1/T1 - 1/T2)*deltaH/R

I used approximate data and some idealizations to calculate the evaporation enthalpy (deltaH) and the phase diagram of P2P. Data used: b.p. at 1atm: 215°C ; b.p. at 14torr: 100.5°C. The deltaH for water was taken from literature (40660 J/mol). The enthalpy was idealized to bee non-temperature dependent (off course, this is not true).

Results:

deltaH(P2P) = 52.86 kJ/mol = 393,87 kJ/kg

Table of calculations:
















temperaturevapor pressuresP[kPa] mixture boils ata:steam distillate composition
T[°C]H2OP2P[kPa]Xb [mol(P2P)/mol]Wc [g(P2P)/g]
00.8320.0040.8350.4%3.1%
101.5660.0081.5740.5%3.7%
202.8250.0172.8420.6%4.4%
304.9000.0364.9360.7%5.1%
408.2070.0708.2760.8%5.9%
5013.3130.13013.4431.0%6.8%
6020.9760.23621.2121.1%7.7%
7032.1870.41132.5981.3%8.7%
8048.2060.69548.9011.4%9.7%
9070.6081.14171.7491.6%10.7%
100101.3251.825103.1501.8%11.8%
110142.6892.848145.5372.0%12.9%


Error: Table contains the text "" between [tr] and the next [td] markup tag in the table row "[tr]>[td]temperature[/td][td]vapor pressures[/td][td]P[kPa] [/td][td]mixture boils at<sup>a</sup>:[/td][td]steam distillate [/td][td]composition[/td][/tr]".

Error: Table contains the text "" between [/td] and the next [/tr] markup tag in the table row "[tr][td]T[°C][/td][td]H2O[/td][td]P2P[/td][td][kPa][/td][td]X<sup>b</sup> [mol(P2P)/mol][/td][td]W<sup>c</sup> [g(P2P)/g][/td][/tr]".




a It is assumed that the vapor pressure of a two phase system like H2O/P2P equals the sum of each phase partial pressures at the given T. Effects of impurities is ignored.
b X is the molar fraction of P2P in the distillate and if ideal gas assumption is used it equals P(P2P)/P.
c Since P2P density is 1.003 g/ml (Fluka catalog) the w/w% showed in the table can also be taken to equal vol% of the P2P phase in the distillate. Therefore if W=22% a 1dl of distillate will be composed of 22ml P2P and 78ml of water. Off course, this is only in theory.


Phase diagram:


Conclusion

Doing the steam distillation of P2P in a reduced pressure does have some influence on the composition of the distillate. Doing a steam distillation at 40°C (~8.3kPa) causes the distillate to contain two times less P2P (from 12.9% to 5.9%). However, due to the faster process and working at a lower temperature, it might bee practical if a compromise is done. Steam distillation between 60 to 80°C might be faster while still reducing the P2P/H2O ratio of the distillate in an acceptable range.
This conclusion is not to bee generalized to all steam distillable compounds since the results are (very!) strongly dependent on evaporation deltaH which is a specific property of each compound. Small differences in deltaH cause huge variations in the steam distillation. More specifically:
- if the molar deltaH is higher than that of the water, vacuum steam distillation causes a lowering of the non-polar fraction content in the distillate;
- if the molar deltaH is lower than that of the water, vacuum steam distillation causes a lowering of the water fraction content in the distillate;
- if deltaH are equal or very similar, no considerable effect should bee noted.

PS: If someone needs a theoretical phase diagram for any other liquid I can easily do some (now that I have the Excel template). All I need is two b.p. of the compound at two different pressures (normal b.p. and a reduced pressure b.p.).



The post has been heavily edited to incorporate the changes brought by the exact b.p./P data supplied by Armageddon in the next post in this thread. New phase diagram and huge differences in the colclusions! Sorry for the inconvenience.


armageddon

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THX
« Reply #22 on: May 31, 2004, 05:03:00 PM »
WOW!

Now THAT'S a complete answer! Thanks a lot nicodem!

(BTW I just estimated the low pressure bp with a nomograph - literature bp of P2P is 215°C at 760 torr and 100-101°C at 14 torr, it's density is 1.0157 g/cm3 at 20°C (merck index) - but I think it doesn't make any significant difference. Vac distillation is just much quicker than normal dist.)

THX!

A


Nicodem

  • Guest
Ops! Post revisited.
« Reply #23 on: May 31, 2004, 08:48:00 PM »
Armageddon, that little change made a huge difference. You probably used a linear approximation to get the b.p. at 60torr? The problem is that the b.p. to vapor pressure is an logarithmic curve. Before the molar deltaH of P2P and water were much more close together. Unfortunately this small change in deltaH caused exponential changes in partial vapor pressures of P2P. :(
Well, at least I learned something new and interesting. Beside this does not mean the vacuum steam distillation of P2P isn’t a good idea. Like you saw, really small changes cause unpredictable conclusions and in practice you always have small changes that the physical chemistry doesn't predict.
I also think that an addition of some salt (CaCl2, NaCl or Na2SO4) in the distillation flask would lower the partial vapor pressure of the water and give a distillate with a much higher proportion of P2P. This in addition with vacuum might bee faster than doing it at a normal pressure. That is, if speed is really so important to you (<-please note the play of words :) ).